The present invention relates to carrier particles for use in a two-component dry-type developer for developing latent electrostatic images to visible images, for use in electrophotography, electrostatic recording and electrostatic printing, and more particularly to carrier particles coated with a silicone resin which is hardened by an organic tin catalyst.
Conventionally, as a developer for use in these fields, the so-called two-component dry-type developer is known, which comprises (i) carrier particles made of, for example, iron particles or glass beads, and (ii) toner particles made of, for example, resins and coloring agents. In the two-component dry-type developer, the particle size of the toner particles is very much smaller than the particle size of the carrier particles, and the toner particles are triboelectrically attracted to the carrier particles and are held on the surface of the carrier particles. The electric attraction between the toner particles and the carrier particles is generated by the friction between the toner particles and the carrier particles. When the toner particles held on the carrier particles are brought near or into contact with a latent electrostatic image, the electric field of the latent electrostatic image works on the toner particles so as to separate the toner particles from the carrier particles, overcoming the bonding between the toner particles and the carrier particles, with the result that the toner particles are attracted toward the latent electrostatic image,so that the latent electrostatic image is developed to a visible image. In the case of the two-component dry-type developer, since only the toner particles are consumed in the course of development, it is necessary to replenish the toner particles from time to time during the development of latent electrostatic images.
Further, it is necessary to triboelectrically charge the toner particles to the desired polarity and with a sufficient amount of charge, and to maintain the amount of the electric charge and polarity thereof for a sufficiently long period of time for use. In the case of a conventional two-component type developer, it is apt to occur that the surface of the carrier particles is eventually covered with the resin contained in and released from the toner particles in the course of the mechanical mixing of the toner particles and the carrier particles in a development apparatus. Once the surface of the carrier particles is covered with the resin, which is generally referred to as the "spent phenomenon", such carrier particles no longer function as active carrier particles capable of substantially charging toner particles for development. As a result, the charging characteristics of the carrier particles deteriorate with time while in use. In the end, it becomes necessary to replace the entire developer by a new developer in the case of a conventional two-component dry-type developer.
In order to prevent such spent phenomenon, a method of coating the surface of carrier particles with a variety of resins has been proposed. For instance, carrier particles coated with a styrene-methacrylate copolymer, polystyrene or a silicone resin are known. Resins which can prevent the spent phenomenon, however, have not been discovered. At one extreme, for instance, carrier particles coated with a styrene methacrylate copolymer are excellent in the triboelectric charging properties. However, since the surface energy of the carrier particles is comparatively high, the carrier particles are easily covered with the resin contained in the toner particles while in use. As a result, the spent phenomenon occurs easily. Accordingly, the life of such developer is not long enough for practical use. Since carrier particles coated with a silicone resin have a small surface energy, the spent phenomenon hardly takes place. However, since the silicone resin itself has a small charging capability of electrically charging toner particles, it cannot be practically used without any modification to the resin. Therefore, when silicone-resin-coated carriers are employed, a polarity control agent capable of charging toner particles with an appropriate quantity of electric charges has to be added to the developer. As such polarity control agent, for instance, metal-complex-containing type monoazo dyes, metal-complex-containing type diazo dyes, and di- or triphenylmethane dyes are known. These dye polarity control agents, however, are expensive and a large amount of the agents is necessary when used in practice. When they are added to the toner and are mixed with the toner for a long period of time, the development performance of the developer gradually deteriorates. Eventually, high image quality cannot be obtained in a stable manner.